[ELF] Update st_size when merging a common symbol with a shared symbol

When a common symbol is merged with a shared symbol, increase st_size if
the shared symbol has a larger st_size. At runtime, the executable's
symbol overrides the shared symbol.  The shared symbol may be created
from common symbols in a previous link.  This rule makes sure we pick
the largest size among all common symbols.

This behavior matches GNU ld. See
https://sourceware.org/bugzilla/show_bug.cgi?id=25236 for discussions.

A shared symbol does not hold alignment constraints. Ignore the
alignment update.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D71161